NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Performance and Reliability of 

Bonded Interfaces for HighTemperature Packaging

Douglas DeVoto

National Renewable Energy Laboratory

Douglas.DeVoto@nrel.gov

303.275.4256

Team members: Paul Paret and Andrew Wereszczak* (ORNL)

* Jointly funded by the OVT APEEM and OVT Propulsion Materials Programs

DOE Vehicle Technologies Office

Advanced Power Electronics and Electric Motors R&D 

FY15 Kickoff Meeting

Oak Ridge National Laboratory

Oak Ridge, Tennessee

November 19, 2014 This presentation does not contain any 

proprietary or confidential information NREL/PR-5400-63003
DOE APEEM FY15 Kickoff Meeting 2

State of the Art

• Current automotive power electronics are transitioning from silicon to 

wide bandgap (WBG) devices to meet cost, volume, and weight targets

• Packaging designs must improve to take advantage of WBG devices’ 

operating parameters:

o Higher operating temperatures

o Higher heat fluxes

o Hot spots

• Current solder alloys do not meet packaging requirements

Traditional Power Electronics Package

Device

Metalized Substrate

Substrate

Attach

Base Plate

Die Attach

Interconnect Encapsulant

Enclosure

Terminal
DOE APEEM FY15 Kickoff Meeting 3

Proposed Technology

• Prior temperature cycling has shown sintered-silver to be 

more reliable than Sn63Pb37 solder as a substrate attach 

material

• Sintered-silver’s thermal performance makes it an attractive 

bonded interface material (BIM) solution:

o High re-melt temperature (962°C)

o High thermal conductivity

0%

5%

10%

15%

20%

25%

30%

35%

0 500 1,000 1,500 2,000 2,500

Delamination

Number of Cycles

Sintered Silver

Sn63Pb37 Solder 1,500 Cycles

2,500 Cycles

1,000 Cycles
DOE APEEM FY15 Kickoff Meeting 4

Challenges/Barriers to Meet Project Goals

• As operating temperatures increase, the coefficient of thermal expansion 

(CTE) mismatch between the substrate and the base plate causes defect 

initiation and propagation in the joining layer

CTE (ppm/°C)

Silicon: 2.6

AlN: 4.5

Si3N4: 3.2

Copper: 16.5

Aluminum: 22.7

Sn63Pb37 Solder: 24.7

Silver: 19.5

• Sintered-silver reliability has not been documented at 200°C conditions 

for the substrate-attach layer

o Oak Ridge National Laboratory’s (ORNL’s) and NREL’s prior experience 

with sintered-silver processing will generate recommended practices 

for synthesis of reliable interfaces

Cooling

Heating
DOE APEEM FY15 Kickoff Meeting 5

FY14 Accomplishments

• Identified threshold at which stress intensity factor is 

sufficient to cause delamination initiation

o The stress intensity factor is a function of the loading amount, 

deformation mode, and the region of interest relative to the crack 

tip deformation

o Crack tip deformation can propagate through three modes:

– Tension, KI

– Shear, KII

– Tearing, KIII

V= da/dN, Crack Growth Rate 

(mm/cycle)

KII, Stress Intensity Factor

V-K Curve

K0 KC

V= da/dN, crack growth rate (mm/cycle)

K = stress intensity factor

-10

0

10

20

-0.40 -0.35 -0.30 -0.25

XZ- Shear stress (MPa)

XZ-Inelastic shear strain

Profile 1

Profile 2

Profile 3
DOE APEEM FY15 Kickoff Meeting 6

FY14 Accomplishments

• Identified threshold at which stress intensity factor is 

sufficient to cause delamination initiation

o Measured delamination rate of FY13 sintered-silver samples

o Modeled stress intensity factor with finite element analysis (FEA) V= da/dN, Crack Growth Rate (mm/cycle)

V-K Curve

K0 KC

V= da/dN, crack growth rate (mm/cycle)

K = stress intensity factor
DOE APEEM FY15 Kickoff Meeting 7

FY14 Accomplishments

• Focused on optimizing and understanding key synthesis parameters for 

sintered-silver:

1. Identified threshold at which stress intensities are sufficient to cause defect 

initiation

2. Evaluated the defect region where a transient delamination rate occurs

3. Evaluated the defect region where a constant slope delamination rate 

occurs

0%

5%

10%

15%

20%

25%

30%

35%

0 1,000 2,000 3,000

Delamination

Number of Cycles

1 2

3

0

1

2

3

4

5

6

7

0 1,000 2,000

Delamination Distance (mm)

Number of Cycles

Quadrant 1

Quadrant 2

Quadrant 3

Quadrant 4

2 1

3 4
DOE APEEM FY15 Kickoff Meeting 8

FY14 Accomplishments

• Processed CTE-mismatched disk samples with various diameter bond pads 

to validate stress intensity factor relationship with delamination initiation

• Invar and copper were selected for round test coupons

o Coupon dimensions are 25.4 mm in diameter, 2 mm in thickness

o Materials were chosen for CTE mismatch

o Surfaces were blanchard ground and metalized with silver

CTE 1

CTE 2

CTE 1

CTE 2

CTE 1

CTE 2

Invar and Copper Test Coupons

Invar Copper

Metalized Invar Metalized Copper
DOE APEEM FY15 Kickoff Meeting 9

FY14 Accomplishments

Quarter symmetry model Crack modeled in interface layer

• Fracture-mechanics-based crack modeling adopted 

for sintered-silver

• Crack size and location needs to be known in advance

• An elliptical crack is built in the area of interest before 

solving

• Mesh re-adjusts to a denser grid around the crack 

accordingly
DOE APEEM FY15 Kickoff Meeting 10

FY14 Accomplishments

K versus distance along crack contour

• K values for each crack mode at a 

given location are obtained

o Stress intensity factor (K) is a parameter 

that describes the stress field near a 

crack tip

• Stress is infinite at the crack tip

• K values at different locations along 

the crack propagation path can be 

obtained
DOE APEEM FY15 Kickoff Meeting 11

FY14 Accomplishments

Quarter symmetry models to replicate crack propagation

• Crack is modeled at greater incremental distances from 

the far corner

• Crack propagates when K > Kcritical

• Geometry is manually changed as propagation cannot be 

modeled
DOE APEEM FY15 Kickoff Meeting 12

FY15 Tasks to Achieve Key Deliverable

2014 2015

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Go/ 

No-Go

Key 

Deliverable

Model stress intensity factor 

with FEA and fit V-K curve to 

disk sample results

Go/No-Go: Do bonds meet minimum strength requirements?

Key Deliverable: Publish V-K curve for sintered-silver

Process CTE mismatched 

disk samples with various 

diameter bond pads

ORNL

NREL

Perform accelerated life 

testing and reliability 

characterization

Process shear-stress 

samples

Perform shear testing
DOE APEEM FY15 Kickoff Meeting 13

CTE 2

CTE 1

FY15 Tasks

• Subject round samples to accelerated temperature testing:

o –40°C to 200°C thermal shock

o 175°C and 250°C temperature elevation

• Monitor delamination rates through acoustic microscopy

-100

0

100

200

300

0 10 20 30

Temperature (°C)

Time (min)

Temperature Test Conditions
DOE APEEM FY15 Kickoff Meeting 14

FY15 Tasks

• Synthesize and shear test initial samples for mechanical characterization 

of sintered-silver

o Attempt to measure residual stress at room temperature

o Estimate stress-strain curves

o Use information to model plastic deformation

• Sintered-silver interfaces are bonded between direct bond copper 

substrates

Shear Test Fixture and 

Sample

Proposed Sample
DOE APEEM FY15 Kickoff Meeting 15

FY15 Tasks

• Evaluate material properties

o Stress-strain curves obtained from shear testing

o Compare temperature-dependent material properties of 

bulk versus sintered-silver

• Establish confidence in stress intensity factor results

o How does ANSYS compute K?

o Onset of plasticity – elastic plastic fracture mechanics

o Develop crack propagation modeling

• Establish V-K curve for sintered-silver
DOE APEEM FY15 Kickoff Meeting 16

FY16 Tasks

• Evaluate the delamination rate of sintered-silver test coupons 

under various pressure requirements, bond pad geometries, 

and surface plating materials

Plating 

Material Ag, Au

Cleaning

None, substrate 

cleaning, preoxidation

Poor Ag Plating

0

5

10

15

20

25

Pressure (MPa)

Recommended Synthesis 

Pressure

Evaluate low- and no-pressure 

sintered-silver materials

Optimize pad geometries for a 

large-area bond pad

Recommend industry standard 

practices for plating
DOE APEEM FY15 Kickoff Meeting 17

Project Summary

Project Duration: FY14 – FY16

Overall Objective: Provide data to support broad industry demand for improving sintered-silver 

reliability.

FY14 Focus: Identify threshold at which stress intensities are sufficient to cause defect initiation.

Deliverable: Publish defect initiation findings for sintered-silver.

Go/No-Go: Can threshold stress before delamination initiation be defined?

FY15 Focus: Evaluate the delamination rate of sintered-silver round test coupons, update sintered-silver material 

properties from shear testing, and develop V-K curves through modeling.

Deliverable: Publish V-K curve for sintered-silver.

Go/No-Go: Do bonds meet minimum strength requirements?

FY16 Focus: Evaluate the delamination rate of sintered-silver test coupons under various parameters and develop 

correlation between interface patterning/degradation and junction temperature rise.

Deliverable: Defect initiation and progression as a function of sintered-silver material and bonding parameters.

Go/No-Go: Adjust bonding parameters to meet minimum shear strength.
DOE APEEM FY15 Kickoff Meeting 18

Technology-to-Market Plan

• This research is on the path to commercialization of high-temperature 

bonded interface materials

o Coordinates with industry suppliers of sintered-silver-based interface 

material

o Addresses key barrier to improve thermal performance of power 

electronics package under high-temperature operation

o Will impact manufacturing parameters of sintered-silver material for 

large-area substrate attachment

• Research directly impacts industry needs for high-temperature attach 

solutions

• Research focuses on achieving targets of a low-cost, high temperature 

(200°C) power electronics package with 15-year rated lifetime
DOE APEEM FY15 Kickoff Meeting 19

Partners/Collaborators

• ORNL: technical partner on sintered-silver samples

• Fraunhofer: modeling collaboration

• Henkel: sintered-silver material guidance

• Heraeus: sintered-silver material guidance

• General Motors: technical guidance
For more information, contact:

Principal Investigator

Douglas DeVoto

Douglas.DeVoto@nrel.gov

Phone: (303) 275-4256

APEEM Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303) 275-4062

Acknowledgments:

Susan Rogers and Steven Boyd

U.S. Department of Energy

Team Members:

Paul Paret

Andrew Wereszczak* (ORNL)

* Jointly funded by the OVT APEEM and 

OVT Propulsion Materials Programs